I. Field of the Invention
This invention relates to servo tracking control by which a tape dispensing apparatus may be maintained in desired spatial or conformal relationship with a contoured surface to which the tape is to be applied. More particularly, the present invention relates to such servo tracking control in which positioning of the dispensing apparatus may be balanced between response to computer position commands and response to servo tracking control signals depending upon the severity of the contour of the surface.
II. Description of the Prior Art
By way of background, a computerized tape-laying machine may include a tape dispensing apparatus which is positionable and movable under computer control relative to a layup tool or the like to which a composite tape material is to be applied to form parts such as aircraft wings, for example. Such a machine includes a horizontal gantry mounted for linear movement above the ground on a machine frame including left and right sidewalls fixedly supported on respective left and right pylons. Mounted for linear movement perpendicularly relative to the gantry is a carriage which movably supports a tape applicator head. The tape applicator head is vertically and rotationally movable such that in cooperation with the gantry and carriage, a tape dispensing shoe of the tape applicator head is movable in a plurality of rectilinear and/or rotational axes under control of a computer program by which to apply several plies of composite material or tape to the layup tool placed between the pylons.
For example, the tape applicator head is movable vertically upwardly and downwardly relative thereto by a Z-axis servo control under program control. Also, the tape applicator head is movable horizontally relative the layup in an X-axis (by movement of the gantry) and in a Y-axis (by movement of the carriage) all under program control. The tape applicator head is further movable under program control along an arc over the layup, for example, along an A-axis. Movement of each member (gantry, carriage, tape applicator head, etc.) is effected by one or more motors under control of respective servo controls. As is well known, commands from a computer control or the like to a servo control will cause the servo control to generate appropriate voltage signals to effectuate rotation of the related motor by which the member is propelled. That is, in response to appropriate commands from the computer, the selected members will thus be driven along the desired linear or rotational axis a desired distance or angle in a desired direction.
Ordinarily, movement of the tape applicator head will result in a corresponding movement of the tape dispensing shoe coupled thereto. Thus, by moving the members as desired, the tape dispensing shoe will follow a desired path and cause various patterns of composite layers to be placed upon the layup tool in a pattern defined by the path which the tape dispensing shoe follows.
In a typical program mode, movement of the various members is effected by motors coupled to servo controls which are under direct control of he computer program. The computer calculates the distance S the head is to be moved and, based upon predetermined feed rates, determines how far each member should move in its respective axis over a predetermined time or interpolation interval. The computer will utilize that information to repeatedly generate change in position commands by which to simultaneously instruct the servo controls to cause the motors to move the various members respective distances in their respective axes during that particular interpolation or iteration interval. Thus, for example, instructing the head to move in one direction 0.01 inch every 10 ms ideally results in effecting movement of the head along the selected axis at a velocity of 1.0 inch/sec. To accomplish same, the servo control generates a voltage signal corresponding to the desired velocity of the appropriate member, which velocity is correlated to the change in position commands from the computer.
Also, as is conventional, a resolver coupled to each drive motor generates a resolver signal which is utilized to indicate to the associated servo control the position of the associated member. Coupled between each motor and related servo control is a drive amplifier to supply motor drive currents in response to the velocity signal from the associated servo control. The motor may also provide a tachometer signal for use by the drive amplifier in a velocity feedback loop as is conventional. The servo controls will each generate a velocity command signal based upon a following error signal which is derived from the actual position of the member (as indicated by the resolver signal) and the desired position thereof (as calculated using the change in position command signal from the computer). Typically, the following error signal is the difference between the actual and desired position signals. The servo control multiplies the following error signal by a gain factor signal to generate the velocity command signal. The velocity command signals are then converted in the servo control to voltage signals and coupled through an associated drive amplifier to the motor to cause movement of the member at a velocity correlated to the following error and gain factor signals. The gain factor signal is selected so that the voltage signal corresponding to the velocity command signal will result in movement of the member at a predetermined velocity correlated to a predetermined following error signal, e.g., 1 inch/min for one-thousandth inch following error signal (1 inch/min per 1/1000 FE). The gain factor signal facilitates correction of known relationships in the gear mechanisms, for example. By controlling motion of each member in the foregoing manner, the tape dispensing shoe can be caused to follow previously defined paths including various contours.
It is desirable that the tape dispensing shoe precisely follow the pattern of axial coordinates as directed by the computer so that the tape will be applied to the layup tool in the desired manner and with the desired conformal relationship between the shoe, tape and layup tool.
Loss of the desired conformal relationship could result in misapplied tape material. As an example, slight misalignment between the layup tool and the shoe could result in the shoe and the tape being vertically displaced slightly above the layup tool rather than in contact therewith resulting in poor application of tape. Alternatively, the shoe could be driven against the layup tool with too much force thus possibly damaging the tape as it is applied.
To overcome problems which might, for example, result from such slight misalignment, the tape dispensing shoe may be permitted to "float" over a limited vertical distance in the Z axis relative a Z axis reference point to which the computer has instructed the tape applicator head (and thus the shoe). The tape dispensing shoe may similarly be permitted to float over a limited arcuate distance in the axis relative an A axis reference point to which the computer has instructed the tape applicator head (and thus the shoe) to move. If there is only slight misalignment, permissible float may be sufficient to overcome problems due thereto. Preferably, however, as the shoe moves over any part of its available float distance (due, for example, to interaction of the layup tool and the tape dispensing shoe) the head is to be repositioned from the positions commanded by the computer to compensate for the extent of movement over the float distance to thereby maintain the desired spatial or conformal relationship between the shoe and the layup tool. In the past, such compensation has been done externally of the computer by a further error signal coupled to the servo driver.
To generate the further error signal, the distance of float over which the head has moved, i.e., difference between the position or reference point to which the shoe was commanded by the computer (e.g., the desired X or A coordinate) and the actual position of the shoe is measured. An electrical signal correlated to the movement over the float distance is then generated. The further error signal is proportional to the aforesaid electrical signal and is applied to the servo motor to further position the head beyond or before the reference position. While this technique of float compensation results in useful servo tracking control between the shoe and the layup tool, the ability to maintain tracking has not been satisfactory for many situations.
Additionally, in some situations movement of the tape applicator head in the X- and/or Y-axes has been under program control while the tape applicator head has been permitted to move in the vertical or Z axis, for example, exclusively in response to vertical changes in the layup tool. In such applications, servo tracking control is not only useful but necessary.
In this type of application, the head is positioned so that it contacts the layup tool. Thereafter, as the tape head moves horizontally across the layup, any vertical inclination or contours, for example, would cause some movement over the Z-axis float distance. That is, because the tape dispensing shoe is permitted to float somewhat, it will generally tend to follow the contour. As with programmed control subject to misalignment, it is desirable to vertically reposition the entire tape applicator head as the dispensing shoe moves horizontally so as to maintain the same spatial or conformal relationship between the head and the layup throughout the traverse of the layup. Moreover, large variations in contour require that the entire tape applicator head move, otherwise the limited distance of float will be exceeded.
Movement of the dispensing shoe in response to a contour variation will result in displacement between the shoe and an initial Z axis reference point as determined when the shoe first contacts the layup (i.e., movement over the Z axis float distance). The further error signal generated in response to the displacement has been applied, as discussed above, to the Z axis servo motor to cause a corresponding adjustment in the shoe position relative the initial Z axis reference point. As the Z axis servo motor repositions the head (to restore the shoe to its nominal position), the Z axis reference point is then also shifted. The corresponding change in vertical spacing between the reference point and the shoe will offset the earlier displacement. If the shoe continues to change position due to the layup tool contour, the reference point will continue to adjust in an effort to maintain the shoe in a desired spatial or conformal relationship with the layup tool. Absent such tracking servo control, the shoe could lose contact with the layup tool or jam into the layup tool surface, neither of which is desirable.
As mentioned, the tape dispensing shoe may also move in an arcuate A axis so as to account for angulation contours in the layup. Movement of the tape head in the A axis has similarly been either under program control with servo tracking as before described or with servo tracking alone.
It will be appreciated that as the severity of the contour increases from less severe or gentle to more severe (e.g., from flat to undulating to having discontinuities) it becomes more difficult to maintain the desired spatial or conformal relationship. For example, at a discontinuity, such as where the slope of the layup changes from upward to downward, if the tape head is not pre-programmed to be adjusted vertically at that location, the head will tend to continue in its upward movement whereby the shoe might disengage the layup for a time resulting in misapplied tape, e.g., inconsistent compaction, because the desired conformal relationship is no longer present. Conversely, if the slope changes from downward to upward, without program control, the tape dispensing shoe could ram the layup tool. With the servo tracking of the prior art, such problems were not adequately avoided. Additionally, the types of contours encountered may change from time to time. For example, slight misalignment problems, as mentioned, may be accounted for by servo tracking alone whereas severe contours may require program control as well. With the servo tracking of the prior art, such variations in work surfaces from job to job were not easily accounted for.